The present invention relates to porous vascular grafts. More particularly, the present invention relates to a porous vascular graft having its innermost surface formed from porous hollow fibers. The porous hollow fibers enhance the ingrowth of endothelial cells which promotes healing of the graft, and as a result ensures the assimilation of the graft into the body.
Vascular grafts are widely used to replace damaged or diseased veins and arteries. Vascular grafts are generally porous to promote the ingrowth of tissue during the healing process. There are numerous types of porous vascular grafts. One particular type of porous vascular graft is formed from a fluoropolymer, and specifically a polytetrafluoroethylene (PTFE) hollow body.
Fluoroplastics, such as polytetrafluoroethylene, are particularly advantageous for preparing vascular grafts because of their chemical inertness. The porous graft is created by expanding a formed fluoroplastic body, typically by stretching the body. The expanded or stretched fluoroplastic, and in particular PTFE body is characterized by a unique microporous node and fibril structure, see U.S. Pat. No. 3,953,566, Apr. 27, 1976 for more detail. While such expanded PTFE bodies possess some of the characteristics desirable for vascular grafts, the fabrication of the porous prosthetic grafts can be somewhat problematic. The problem of using fluoroplastics for vascular grafts is due to the fact of the considerable difficulty in making an article porous, and keeping it so, yet providing it with adequate strength.
Complicated, expensive processes have been devised to achieve porosity, yet retain strength. Such processes include, for example, adding a leachable material to the PTFE prior to forming, and subsequently leaching the material out of the formed article with a solvent.
The chemical inertness of fluoroplastics, such as PTFE, although a very desirable property, can be disadvantageous in some respects. For example, many materials are not compatible or miscible with PTFE, thus blending other property-enhancing materials with PTFE can be difficult. This incompatibility may cause, for example, delamination or bleeding of blended materials.
Additionally, processing of fluoropolymers has proved problematic due to their high molecular weight and melt viscosity. Certain fluoroelastomers, including poly-(tetrafluoroethylene-co-propylene). disclosed in U.S. Pat. No. 4,463,144, have a high molecular weight and hence can only be extruded with difficulty. Thus researchers have sought other materials from which vascular grafts may be formed.
Another type of porous vascular graft is one formed from one or more polymeric fibers wound about a mandrel. These fibers are wound in such a manner to form a solid structure in which the pores are defined as the spacing between adjacent fibers. Generally, this type of graft is formed from numerous fibers wound in a selected pattern about the mandrel. The advantage of this type of vascular graft is that the polymer composition from which the individual fibers are prepared can be selected to achieve different physical and chemical characteristics.
There are numerous methods of preparing vascular grafts from individual fibers. One particular method for preparing a synthetic porous vascular graft from individual fibers involves the electrostatic spinning of an organic polymer fiber about a rotating mandrel. This type of method is taught in European Patent Publication 95940. The spun fibers bind to one another with the porous structure being defined by the interconnected openings between the adjacent fibers. The porosity of the graft can be controlled by varying the mandrel rotating speed as the fibers are being wound about the mandrel.
Similar processes for preparing porous vascular grafts involving the spinning of fibers about an electrostatically charged mandrel are described in Annis, et al. (Transaction ASAIO, Vol. 24 1978) and U.S. Pat. No. 4,323,525. In the former process the fibers are formed by discharging a fiber forming polymer, in solution, from nozzles directed at the spinning mandrel. The electrostatic charge promotes the attraction of the fibers to the mandrel. The later process includes the use of a mandrel composed of a core and a removable sheath. The removable sheath promotes the ease of removing the formed fibrous tube.
U.S. Pat. No. 4,475,972 describes a process for preparing a vascular graft by winding polymeric fibers on a mandrel without electrostatic spinning. The overlying fibers are simultaneously bonded together. In this process, the porosity is created by varying the diameter of the fibers and the winding angles. A similar process is disclosed in Great Britain Patent Application No. 2115776, with the wound fibers bonded together using an adhesive or heat treatment. U.S. Pat. No. 4,355,426 describes a small pore flexible vascular graft having a luminal pore surface in fluid flow communication with the network of interconnected sphere-like pores.
While the above described vascular grafts provide adequate pore size to promote growth of tissue during the healing process, the pore structure of such grafts is relatively large. The relatively large pore structure promotes bleeding, that is the passage of the blood through the graft. This causes the potential of the retention of red blood cells within the graft. Further, large pore openings in a graft usually promote collagenous tissue ingrowth which may render the graft inflexible. This disadvantage with the pore size of presently available vascular grafts is particularly acute during the initial healing stages after the implantation. Specifically, the relatively large pore size reduces the potential of a satisfactory healing due to this potential of bleeding.
Another disadvantage with presently available vascular grafts concerns the luminal or inner graft surface. In accordance with conventional techniques, such as taught in U.S. Pat. No. 4,475,972, the resulting graft luminal surface is relatively rough and loose surface. Even with electrostatic spinning the inner surface of the graft can be rough and so porous so as to entrap various blood elements, leading to undesired acute thrombosis.
It would also be advantageous if presently available vascular grafts include the ability of providing a direct delivery of various types of drugs to the suture site. That is, if vascular grafts were constructed in some fashion to be able to temporarily store and then release a drug, e.g. an anticoagulant, into the blood. This could be useful in promoting the healing process, or eliminating the undesired thrombogenicity.